Scientists have discovered a "bridge" made of dark matter that
connect clusters of galaxies:

A ‘finger’ of the Universe’s dark-matter skeleton,
which ultimately dictates where galaxies form, has been observed for
the first time. Researchers have directly detected a slim bridge of
dark matter joining two clusters of galaxies, using a technique that
could eventually help astrophysicists to understand the structure of
the Universe and identify what makes up the mysterious invisible substance
known as dark matter.

According to the standard model of cosmology, visible stars and
galaxies trace a pattern across the sky known as the cosmic web, which
was originally etched out by dark matter — the substance thought
to account for almost 80% of the Universe’s matter. Soon after
the Big Bang, regions that were slightly denser than others pulled in
dark matter, which clumped together and eventually collapsed into flat
‘pancakes’. “Where these pancakes intersect, you get
long strands of dark matter, or filaments,” explains Jörg
Dietrich, a cosmologist at the University Observatory Munich in Germany.
Clusters of galaxies then formed at the nodes of the cosmic web, where
these filaments crossed.